After you've completed the Gerudo Fortress there are quite a few side quests you can complete. Here they are:
After all that, it's time to get on with the main adventure.You should be in the Gerudo's Fortress. Head as north as you can go, and you should see a gate that is currently locked. Climb the ladder and talk to the Gerudo Theif dressed in all white. She will tell you that you need to cross the river of sand (that you can't walk across) and follow a guide (that you can't see). This would sound hard to any normal kid, but your not a normal kid! Your Link! And your here to save Hryule! She'll open the gate and off you go. Your now entering the Haunted Wasteland. Walk off into the distance.
The first thing you come across is the "river of sand". There's a box on the other side, so now that you have a Longshot you can target the box and grapple to the other side.
Now all you have to do is follow the flags. You don't need your Lens of Truth yet. Walk to a flag, stop, find the next flag, then continue. Don't get lost, it's a pain in the ass to have to start again and yes, it is easy to get lost.
Once you walk between two flagstafs you'll come across a little rock structure. There is a Gold Sulltula in the bottom, and if you light both the torches you'll recieve 50 rupees. On the top of the structure you'll find a block that says use the eye of truth. Pull out your Lens of Truth and turn it on.
This ghost will lead you across the Haunted Wasteland. Just leave the Lens of Truth on and follow him. It's not too hard, even though he swerves and weaves. Once you find the double flagstafs you're there.
Welcome to the Desert Colossus.